Zeolite adsorption is one of the best available technologies for gas separation. We use molecular simulation to produce the data of adsorption of n-butane and i-butane and their mixture at different temperatures and pressures in different zeolites. In BEA, MOR, CFI, ISV, and BOG the amount of adsorption of i-butane is higher than n-butane, but the case is just the contrary in MFI, MEL, TER, and TON. The heats of adsorption decrease with temperature increasing for all investigated cases except for i-butane in MFI and MEL. The IAST (ideal adsorbed solution theory) is used to predict the n-butane/i-butane mixture adsorption. For the systems of BEA, MOR, CFI, and ISV, IAST theory provides predictions that are in good agreement with simulation. We propose SACIAST (surface area corrected IAST) by introducing a correcting parameter C-r into IAST to predict the adsorption of mixture in MFI, MEL, TER, TON, and BOG since the original IAST failed to predict the mixture data in these zeolites because of the pore structure of these zeolites. The SACIAST we proposed is a new modified theory based on IAST which has never been reported in the literature before, and it predicts the mixture adsorption very well.